1. Technical Field
The application is related to methods and systems for inspecting satellites with inspection vehicles that travel a path around the satellite to be inspected.
2. Related Technology
Artificial satellites in orbit around the earth can occasionally have problems that require a visual inspection to detect and diagnose. A small vehicle can be sent to move in a path around the satellite to take photographs and inspect or repair the larger satellite.
A satellite (the secondary) circumnavigating another satellite (the primary) in order to inspect it for possible damage or failure will be guided by two goals: first, to avoid collisions with the main satellite, and second, to pass through certain directions (or perhaps, all directions) from the primary from which it is desirable to have a view; a stuck deployable might be imaged for diagnosis and repair on the ground, or perhaps an all-over surface inspection is necessary. If the primary has a protuberance like an antenna or solar panel, and the inspection needs to be at a close distance (on the order of meters), then it may be necessary to have a complicated trajectory in order to meet both conditions. Techniques for planning trajectories for orbital maneuvering have been used successfully for many years, but these techniques do not generally deal with obstacle avoidance. In the last decade, however, spacecraft proximity operations has increased in importance, and consequently techniques for safely operating spacecraft in close proximity to each other have been developed.
Some of these techniques have taken classical astrodynamics as their starting point. Such algorithms typically aim to produce either natural motion trajectories (governed primarily by orbital dynamics) or forced motion trajectories (governed primarily by on-board spacecraft thrusting) that maintain enough distance between the co-orbiting bodies that collisions are impossible. Implicit in this approach is that the co-orbiting bodies' geometry is unimportant; essentially, the bodies are treated as spheres that circumscribe the real geometry of the spacecraft in question. A collision-free trajectory is then one in which the circumscribing spheres do not intersect. This approach has many advantages.
In contrast, the terrestrial robotics community has treated trajectory planning very differently. Robotic trajectory planning is typically concerned with finding collision-free trajectories in highly cluttered or confined environments; one canonical trajectory planning problem in terrestrial robotics entails a mobile vehicle operating inside an office building. Trajectory planning for spacecraft proximity operations based on the classic terrestrial robotics approach thus have the ability to plan much closer maneuvers than those based on classic astrodynamics.
Some approaches are described in U.S. Patent Application Publication No. 2007/0179685 to Milam et al. and 2009/0132105 to Paluszek et al.
Relative motion about a primary in circular orbit in terms of centered relative orbital objects is described in L. M. Healy and C. G. Henshaw, “Passively safe relative motion trajectories for on-orbit inspection”, AAS 10-265, pp. 2439-2458, (2010), the entire disclosure of which is incorporated herein by reference.